For switching laser pulses, optical switches and/or modulators can be used and may be arranged either inside or outside a laser resonator in a beam path. Essentially, optical modulators may be differentiated between electro-optic modulators and acousto-optic modulators. A significant factor in the use of optical modulators is the achievable switching time. In acousto-optic modulators, the achievable switching time is generally defined by the sound velocity and the laser beam diameter. In some cases switching flanks of acousto-optic modulators are too slow to effectively suppress pre-pulses and/or post-pulses, for example, at a short distance from the main pulse. In comparison therewith, electro-optic modulators have been recently developed to achieve higher clock speeds. Thus, electro-optic modulators may be able to replace acousto-optic modulators in specific laser applications, as they are characterised by substantially shorter, electronically induced switching flanks.
Electro-optic modulators are generally designed so that they have a Pockels cell as an actual optical switching element with modifiable optical properties and a polarization-selective element, such as, for example, a reflecting analyser with unmodifiable optical properties. A Pockels cell generally has a birefringent crystal which is directed in an appropriate manner towards an incident monochromatic and polarized light beam and to which an electrical voltage is applied in the order of several 100 V up to several kilovolts. In combination with the polarization-dependent optical element, the Pockels cell is able to:                (a) switch on or switch off the light depending on the electrical voltage applied thereto, and/or        (b) to deflect the light on two different paths through an optical system.        
The Pockels cell may be switched to-and-fro by a suitable switchable high voltage supply between two states in which the laser beam emerging from the Pockels cell (e.g., with polarization directions located perpendicular to one another) is polarized in a linear manner. The voltage that is required in order to achieve the two aforementioned states is a function of the crystal parameters and the wavelength of the light to be switched. Applications of Pockels cells exist in which the Pockels cells have to be switched on and off rapidly such that one or both of the transition times have to be in the range of a few nanoseconds. Where merely one of these transition times has to be short (e.g., either the switching-on or switching-off process), the other transition time may be in the range of microseconds.
Such an electro-optic modulator constructed from a Pockels cell and a suitable switchable high voltage supply may be used, for example, in order to switch optically short laser pulses having a duration of a few nanoseconds (ns) or ultra-short laser pulses having a duration of picoseconds (ps) and/or femtoseconds (fs) (e.g., to alter the intensity or the beam direction of the laser pulses). Such ultra-short laser pulses may be generated by a mode-locking method known to those skilled in the art, for example. Thus laser beam sources for ultra-short pulses, in principle, have relatively high repetition rates (e.g., typically 40-200 MHz for solid state lasers) and low pulse energy (e.g., typically 0.1-50 nJ). If individual pulses or pulse groups of ps-laser pulses and/or fs-laser pulses are required, frequently a Pockels cell is used in order to select said pulses. In this case, initially the voltage has to be fully switched on between two pulses (e.g., which the laser beam source typically transmits at a time interval of 5-25 ns) in order to be fully switched off again after allowing a single laser pulse through 5-25 ns later.
In many applications, therefore, it is a case of achieving more rapid switching times. However, there are also situations in which it is advantageous if at least one of the two switching times is slower than might be possible by the switching element itself. A Pockels cell, for example, may be used inside a laser resonator in a manner known per se as a cavity dumper. In such an embodiment, it may be desirable to make the pulse, which is decoupled from the resonator by the cavity dumper, longer and to reduce the peak intensity. This may be achieved by extending the switching times of the control circuit of the Pockels cell. Accordingly, high voltage circuits may be required in which at least one of the switching flanks is made slower than the switching elements would normally permit.